Striking differences exist between countries in the age-specific incidence of breast cancer, but the causes of these differences are unknown. This application requests resources to carry out a pilot study to examine the possibility that mammographic densities, a strong risk factor for breast cancer within the population, are also related to differences in risk of breast cancer between populations. The hypothesis that motivates this proposal is that Caucasians, at higher risk of breast cancer, will have a greater volume of mammographically dense breast tissue, indicating a greater quantity of stromal and epithelial tissue than recent Chinese migrants to Canada, at lower risk. The purpose of the study proposed here is carry out a pilot study to examine the feasibility of testing this hypothesis. We have no previous data on mammographic volumetric measures to estimate the magnitude of any difference between Chinese and Caucasian women, or of the variance within these groups, on which to base sample size calculations. The main aim of this proposal is to obtain estimates of the values of the mean and variance of the volume of total breast tissue and dense tissue in recent Chinese migrants (N=50) and Caucasian women (N=50) in Canada aged 50 years or more, and to estimate the magnitude of any difference in these measures between these two groups. To measure the volume of mammographically dense breast tissue we must acquire images prospectively under controlled conditions and we have developed a calibration device and a thickness gauge that will be incorporated into the field imaged during mammography to assess tissue composition and thickness. The general method proposed is a cross- sectional study in two groups of women aged 50 years or greater, that have greatly different risks of breast cancer. One group will be Caucasian, and the other recent migrants to Canada born in China. All subjects will be recruited from the Toronto centers of the Ontario Breast Screening Program (OBSP). Information about selected risk factors and anthropometric variables will be collected by telephone interview. Mammograms will be obtained and digitized, and the volume of the breast and of dense tissue determined.